1. Field of the Invention
The present invention relates to an optical disk apparatus, particularly to the optical disk apparatus which is suitably used to record information in a so-called Low-to-High type optical disk in which a reflectance of a recorded portion is higher than that of an unrecorded portion.
2. Description of the Related Art
Currently, HD DVD-R (High Definition Digital Versatile Disc-Recordable) is being prepared for the market. In HD DVD-R, there is proposed a so-called Low-to-High type optical disk in which the reflectance of the recorded portion is higher than the reflectance of the unrecorded portion.
In such optical disks, because a recording layer is made of an organic coloring material, the Low-to-High type optical disk is easily affected by light from the outside compared with a High-to-Low type optical disk in which the recording layer is made of an inorganic material. On the other hand, at least 1000000-time reproductions are demanded in the HD DVD standard. Even in the Low-to-High type optical disk, it is necessary to ensure high reliability for recording information. Therefore, in the optical disk drive apparatus, it is necessary that a laser power be properly set during recording such that degradation of a recording film property and cross erase are not generated due to excessive light irradiation.
FIGS. 12A and 12B schematically show a recording operation when the information is recorded in the Low-to-High type HD DVD-R. As shown in FIG. 12, as with the High-to-Low type HD DVD-R, the recording is performed only to grooves in the Low-to-High type optical disk. Because the optical disk is not rewritable, the recording is sequentially performed to the track one by one from an inner circumference to an outer circumference of the optical disk.
As shown in FIG. 12B, when the recording is finished to a groove (G1), the recording is performed to a groove (G2) adjacent to the groove (G1). In this case, the recorded groove (G1) is partially irradiated with a periphery of a light spot when the recording is performed to the groove (G2). Laser beam intensity in the light spot concentrates on a central portion of the light spot. Therefore, even if the recorded groove (G1) is partially irradiated with the periphery of the light spot, a recording mark is not formed on the groove (G1).
However, at this point, the laser beam leaks slightly into the groove (G1). The leakage of the laser beam affects the recording layer of the groove (G1) to generate the degradation of the recording layer in terms of light stability. Additionally, when the groove is irradiated with the high-power laser beam in the recording operation, the degradation is generated in the recording film property, which possibly results in a problem of reproduction durability.
Thus, in the Low-to-High type HD DVD-R, it is necessary that the setting of the recording laser power be studied from the standpoint different from the High-to-Low type HD DVD-R.
Generally the recording laser power in a write once read many optical disk is set to a power with which a reproduction signal property becomes the best. For example, in the High-to-Low type HD DVD-R, the recording laser power is determined by a test write performed to a test area such that PRSNR (Partial Response Signal to Noise Ratio) becomes the maximum or a reproduction error rate becomes the minimum, and the laser power is set to record main information.
According to the above method, it can be predicted that the reproduction signal property becomes the best immediately after the recording. However, when the method is directly applied to the Low-to-High type HD DVD-R, the degradation of the recording film property may be generated as described above. Therefore, the sufficient signal property is not possibly obtained before the number of reproduction times defined by the standard. According to the verification performed by the inventor, in the case where the recording is performed with the laser power in which the PRSNR becomes the maximum, PRSNR of the reproduction signal is rapidly degraded when the number of reproduction times reaches about 700000 to about 800000 times, and the 1000000 times reproduction operations defined by the standard cannot be realized. The detailed verification will be described in the following embodiments of the description.
The degradation of the recording film property becomes remarkable in the area which is frequently irradiated with the laser beam. For example, the degradation of the recording film is easily generated in the areas, such as a data lead-in area and an area in which file information is stored, which is read in each time the optical disk is reproduced. However, when the areas cannot be produced, the information necessary to reproduce the main information cannot be obtained. As a result, the information in the whole optical disk cannot be reproduced.